The present invention relates to a bookbinding apparatus to make a booklet by coating adhesive on a spine of a sheet bundle and to a bookbinding system equipped with the aforesaid bookbinding apparatus and with an image forming apparatus that forms images on a sheet.
A bookbinding apparatus for binding a sheet bundle composed of a plurality of sheets through gluing which is conducted by moving a coating device along a spine of the sheet bundle has been developed as a small-sized binding apparatus.
In particular, the aforesaid bookbinding apparatus has been developed as s small-sized bookbinding apparatus to be used for a bookbinding system wherein an image forming progress for forming an image on a sheet with an image forming apparatus and a bookbinding progress for binding sheets are conducted in a continuous progress.
For example, in Japanese Patent Publication Open to Public Inspection No. 2006-346984, there is proposed a bookbinding apparatus for conducting coating by moving a coating device along a spine of a sheet bundle wherein coating is conducted through first coating operations by bringing the coating device into contact with a spine of the sheet bundle under strong contact pressure and through second coating operations under weak contact pressure.
The bookbinding apparatus in Japanese Patent Publication Open to Public Inspection No. 2006-346984 conducts the aforesaid first and second coating operations for the purpose of making adhesion between sheets constituting a sheet bundle to be sure and of making adhesion of a cover to be sure.
By changing a thickness of coating of an adhesive layer formed on a spine of a sheet bundle depending on a thickness of a booklet, it is possible to produce a high-quality booklet, and thereby the quality is stabilized.
The reasons for the foregoing are as follows.
In the case of a thick booklet, it is necessary to make a coating thickness to be thick for obtaining necessary adhesive strength. However, if a coating thickness is made to be thick in the same way in the case of a thin booklet, adhesive is squeezed out as side adhesive when wrapping with a cover, resulting in excessive adhesive and poor binding.
On the other hand, when a booklet is thin, sufficient adhesive strength can be obtained even when a coating thickness is relatively thin.
As is suggested in Japanese Patent Publication Open to Public Inspection No. 2006-346984, changing coating conditions depending on a thickness of a sheet bundle is a powerful technique to solve the aforesaid problems. In other words, by changing a coating amount of adhesive depending on a thickness of a sheet bundle, the problems mentioned above can be solved to a certain extent.
However, when conducting coating control based on a thickness of a sheet bundle, a thickness of a formed adhesive layer is changed for each sheet bundle, and an external form of a sheet bundle or an external form of a booklet prepared by joining a cover on a sheet bundle shows lack of uniformity, which is a problem.
The lack of uniformity of the sheet bundle or of the booklet is especially remarkable in the following occasions.
For changing a coating weight depending on a thickness of the sheet bundle, there is employed a controlling method wherein a prescribed thickness represents a threshold value, and when a thickness is the threshold value or higher, a coating amount is increased, while, when a thickness is less than the threshold value, a coating amount is decreased.
When binding processing is continued for sheet bundles each having a thickness close to the threshold value, there is sometimes an occasion wherein a judgment for control of a coating thickness is changed for every sheet bundle even when the number of sheets is the same, resulting in formation of adhesive layers each being different for every sheet bundle.
FIG. 1 shows schematically situations of the prepared booklets wherein adhesive layers which are different in terms of a thickness are formed for every sheet bundle even when the number of sheets in a sheet bundle is the same, as described above.
As a result of adhesive layers N1-N4 of booklets P1-P4 which are different from each other in terms of a thickness, when the booklets P1-P4 are aligned at the reference position of their spines, ends Q1-Q4 of the booklets show lack of uniformity.
In other words, as a result of control of coating amount, there is caused a problem that consistency of booklets is lowered and quality of booklets is lowered.